Hi Jacqui
The information for Fanny's birth registration was from a transcription of the register.
It's unlikely that there is more info on the original but it would be worth finding out.
Sometimes notes are added to an entry especially if there is a discrepancy such as no father's name!
As Margaret would have just been in her late teens then it is likely the birth would have been illigitimate and possible that she was just working or visiting Ballyness.
It was obviously important enough though to quote the exact townland and not the parish or nearest large town.
There were no Stewarts or McClaffertys shown in Ballyness on the Griffiths Valuations of about 1856.
The only one of either name in the 1901 Census is Betty Stewart, a widow aged 87yrs.
[ see:
http://www.census.nationalarchives.ie/reels/nai000654466/ Household #15; Landholder #25 - Betty Stewart
Private Dwelling; Stone; Thatch; 2 Rooms; 2 Windows; 3rd Class; 1 Cow House
Betty STEWART, Head; Protestant Episcopalian; Cannot Read; Age 87yrs.; Farmer; Widow; Born - Co. Donegal; Speaks Irish & English]
None show in the 1911 Census.
I've noticed on another posting you give Margaret's mother as Elizabeth McCAULEY. I wonder is that the widow Betty above?
The Valuation books might show the preceding and subsequent ownership of Betty's holding.
Best of luck with your research.
Henry